1. Field of the Invention
The present invention relates generally to optical integrated devices and optical control devices, and more particularly to an optical integrated device having optical waveguides different in refractive index difference coupled to each other, and to an optical control device using the same. Examples of the optical control device include an optical delay device, an optical switching device, an optical routing device, an optical information processor, a light emitting device, a light receiving device, and a non-linear optical device.
2. Description of the Related Art
Optical integrated circuits employing a planar lightwave circuit (PLC) as a basic interconnection element are capable of advanced optical signal control, and are principal components in optical communications. Many of the optical circuits employed in these optical integrated circuits have a waveguide structure confining light in a core using total reflection due to the difference in refractive index between the core and a cladding. Planar waveguides are interconnection members for optical control devices, and conventionally, those formed of a silica-based material have been employed. The core-cladding relative index difference is small in the silica-based waveguide. Therefore, the silica-based waveguide has a large cross section size of 5 to 10 μm in order to satisfy single mode conditions, and has a large radius of curvature of 1 to 25 mm in the case of bending a waveguide and changing the propagation direction. As a result, the silica-based waveguide is large in device size, and therefore, is not suitable for high integration.
On the other hand, studies have been made of a fine optical control device that employs as the core of an optical waveguide a material having an extremely high refractive index compared with the conventional silica-based materials. For example, by employing a semiconductor material having a refractive index of 3 or higher or a glass material having a refractive index of approximately 2 as a core, and employing air as a cladding, it is possible to achieve a relative index difference of approximately 40% or higher. This makes it possible to achieve significant miniaturization in device size compared with common silica-based PLC devices. Specifically, the bend radius that enables lossless propagation is a few μm, which is less than or equal to approximately one-thousandth of that of the conventional silica-based waveguide. The optical waveguide that has an extremely high relative index difference compared with the conventional total reflection optical waveguide, specifically a relative index difference of 10% or higher, is referred to as “high index contrast (HIC) optical waveguide.”
Various optical control devices have been proposed using such HIC waveguides. As an example of employing silicon as a core material, an ultra-small optical branch device is proposed in [Non-Patent Document 1], and a wavelength filter using a Bragg grating is proposed in [Patent Document 1]. In addition, there is also proposed an optical control device using a photonic crystal waveguide formed of a high refractive index material. The photonic crystal forms a photonic band gap, or a forbidden band of photons, with a periodic structure of about a wavelength of light, and shows a peculiar effect because of strong dispersiveness. In these respects, the photonic crystal is counted on as a micro optical integrated circuit or a new function optical device. For example, a minute path change device and a switching device that performs switching by performing refractive index modulation on a device material are proposed.
By thus forming an optical waveguide of materials between which there is a great difference in refractive index, it is possible to make individual optical devices extremely minute in size. As a result, a future large-scale optical integrated circuit can be expected to have a compact configuration.
However, although a total reflection linear waveguide can perform lossless light propagation in principle, propagation loss is inevitably caused by light scattering due to the roughness of the side faces of the waveguide (core) generated at the time of its fabrication. As the relative index difference of an optical waveguide is greater, the effect of the relative index difference is more conspicuous in the propagation loss. In general, the propagation loss in a linear optical waveguide is proportional to a relative index difference to the 2.5th power. Accordingly, there is a problem in that in the case of configuring an optical integrated circuit formed of multiple optical control devices of an optical waveguide type using only HIC optical waveguides, or in the case of including a linear portion in the device structure, the insertion loss of the entire integrated circuit is extremely great.
Therefore, in order to solve this problem, a waveguide of low refractive index difference and a waveguide of high refractive index difference are stacked in layers different in a vertical direction in [Non-Patent Document 2]. The low index difference waveguide is in charge of signal interconnection, and at the time of performing control such as path changing on a light signal, the light is transferred to the layer formed of the high index difference waveguide including a curved waveguide, where optical control is performed in an extremely minute region.
[Non-Patent Document 1] Sakai, A. et al., “Low Loss Ultra-Small Branches in a Silicon Photonic Wire Waveguide,” IEICE Trans. Electron., Vol. E85-C, No. 4, pp. 1033-1038
[Non-Patent Document 2] Kokubun, Y., “Photonic Wavelength Router Using Microring Resonator,” Oyo Buturi, Vol. 72, No. 11, pp. 1369-1373 (2003)
[Patent Document 1] Japanese Laid-Open Patent Application No. 2004-070015
However, in realizing such a layered optical waveguide device, there is a problem in that the fabrication process is complicated and that it is very difficult align an optical interconnection and an optical control part.